This invention relates to a piezoelectric single crystal wafer for surface acoustic wave or leaky surface acoustic wave devices and a method for preparing the same.
Surface acoustic wave (SAW) and leaky surface acoustic wave (LSAW) devices are circuit elements adapted to transduce electrical signals into SAW or LSAW for signal processing. They are used in filters, resonators and delay lines. In particular, SAW and LSAW devices enjoy a dramatically increasing demand as filters for the current widely spreading mobile communication. Among various wafers used therefor, lithium tantalate single crystal wafers are most often utilized because of their good balance of characteristics. There is an increasing demand for further improvements in quality, especially in the uniformity of SAW and LSAW velocities on the wafer surface and between wafers.
SAW and LSAW devices are generally fabricated by growing a piezoelectric single crystal by a suitable single crystal growth method, for example, growing a lithium tantalate single crystal by the Czochralski method. The single crystal is worked into a cylindrical rod and sliced into a wafer having a desired crystal face orientation. A surface of the wafer on which electrodes are to be formed for transmitting and receiving SAW or LSAW (this surface is herein referred to as wafer surface) is polished to a mirror finish. Electrodes composed primarily of aluminum are formed on the wafer surface in a predetermined orientation. Finally the wafer is cut into chips.
The performance of SAW and LSAW devices depends on the material used, crystallographic orientation, electrode design, fabrication technique and various other factors. One important performance factor associated with SAW and LSAW devices is a work-affected surface layer extending from the surface to a depth of about several tens of microns. The relationship of SAW to the work-affected surface layer and the method of polishing SAW single crystal wafer while taking into account the work-affected surface layer are reported in the following articles.
Kimura et al., Shingaku Giho, US75-56, 17-23, 1975, after making a survey on the relationship of a work-affected surface layer and the Q value of SAW using quartz, report that the presence of a noticeable work-affected layer such as secondary cracks to a depth of half wavelength reveals as a difference of Q value.
T. Kimura et al., J. Appl. Phys., 50 (7), 4767-4772 (1979), after making a survey and analysis on the relationship of a work-affected surface layer and a propagation loss of SAW using quartz, report the relationship of the depth and quantity of microcracks to the propagation loss.
As to the method of polishing a lithium tantalate single crystal wafer for SAW, like the method used for the working of silicon single crystal wafers, a method involving lapping and polishing with a colloidal silica polishing fluid is known (SAW Technology 150th Committee of Japan Society for the Promotion of Science Ed., SAW Handbook, 296-298, 1991). What is required in the polishing step is to establish a mirror finish and to completely remove a strain layer resulting from the lapping step. It is described that since lapping usually uses abrasive grains of less than 15 xcexcm and thus produces a strain layer of less than 10 xcexcm thick, polishing to a depth of 10 xcexcm at most is sufficient.
With respect to the relationship of a work-affected surface layer to LSAW and the method of polishing a single crystal wafer for LSAW, no reports are known.
Further, the SAW velocity and its uniformity have been discussed in connection with the composition and the cutting and propagating directions of a wafer. See SAW Technology 150th Committee of Japan Society for the Promotion of Science Ed., SAW Handbook, 289-302, 1991, and Aikawa et al., Autumn Meeting of the Electronic Information Communication Society, 19, 1994.
However, engineers are now concerned with deviations of SAW and LSAW velocities within wafer surface and between wafers, which are believed to be not accounted for merely by the composition and the cutting and propagating directions. It is strongly desired to analyze the cause of such deviations and find a solution.
An object of the invention is to provide a novel and improved piezoelectric single crystal wafer for SAW or LSAW devices, having a significantly reduced percentage of rejects caused by a deviation of SAW or LSAW velocity. Another object of the invention is to provide a method for preparing the piezoelectric single crystal wafer.
We made a survey on the relationship of a work-affected surface layer to a deviation of SAW or LSAW velocity. One of the reasons why we paid attention to this relationship is that the relationship of a work-affected surface layer to the Q value and propagation loss of SAW had been reported, but not the relationship of a work-affected surface layer to a deviation of SAW or LSAW velocity. As another reason, it is believed that a work-affected surface layer other than secondary cracks must be considered before the current quality requirement can be fulfilled. Although the current quality requirement apparently differs from the past quality requirement, the point of view and the processing method remain unchanged from the past technology. Finding a solution in this regard is a challenge. We speculated that the state in proximity to the surface where the energy of SAW or LSAW concentrates, that is, the state of a work-affected surface layer is predominant. The term xe2x80x9cwork-affected surface layerxe2x80x9d is a surface layer having strain induced by working as well as secondary cracks. Continuing the research, we have found that the half width of an x-ray rocking curve is useful for evaluating a work-affected surface layer. At the initial stage of our research, the work-affected surface layer was evaluated in terms of etching. There was found no evidence that the etching pattern is correlated to a deviation of SAW or LSAW velocity. Then we supposed that the x-ray rocking curve is effective as a method for evaluating the state of a work-affected surface layer. As a result, we have found that the half width of an x-ray rocking curve is correlated to a deviation of SAW or LSAW velocity. We have also found that heat treatment is effective for improving a deviation of SAW or LSAW velocity. The present invention is predicated on these findings.
Accordingly, in a first aspect, the invention provides a piezoelectric single crystal wafer for SAW or LSAW devices, having an x-ray rocking curve half width of up to 0.06xc2x0 on the wafer surface on which electrodes are to be formed for transmitting and receiving SAW or LSAW. The piezoelectric single crystal wafer is typically a lithium tantalate single crystal wafer.
In a second aspect, the invention provides a method for preparing a piezoelectric single crystal wafer for SAW or LSAW devices, comprising the steps of polishing a surface of a piezoelectric single crystal wafer and heat treating the wafer such that the wafer has an x-ray rocking curve half width of up to 0.06xc2x0 on the wafer surface.
According to the invention wherein the half width of an x-ray rocking curve on the wafer surface on which electrodes are to be formed for transmitting and receiving SAW or LSAW is set at or below 0.06xc2x0, frequency variations during device manufacture can be minimized. According to the method of the invention wherein polishing is followed by heat treatment, there can be obtained a SAW or LSAW device piezoelectric single crystal wafer having a minimized deviation of SAW or LSAW velocity within the wafer surface and between discrete wafers. The percentage of rejects caused by a deviation of SAW or LSAW velocity during device manufacture is significantly reduced.